1. Field of the Invention
This invention relates in general to methods for reducing hydrogen sulfide concentrations in subterranean well fluids, and more particularly to methods of reducing hydrogen sulfide concentrations in subterranean well fluids by means of chlorine dioxide.
2. Description of the Prior Art
It is well known that hydrogen sulfide present in well fluids is a serious problem in the process of drilling and treating of wells. The presence of hydrogen sulfide also creates problems in the production steps when the well has been completed. Most significantly, hydrogen sulfide gas is extremely toxic such that those around the well or the produced well fluids must avoid breathing these toxic fumes. Often the toxic hydrogen sulfide is carried to the surface by drilling mud and this mud releases the toxic gas to the atmosphere. Of course, this makes the area around the well extremely hazardous. Furthermore, hydrogen sulfide is also a very corrosive agent such that it damages the tubing of the well and the containers which hold the produced well fluid. Also, hydrogen sulfide is a very reactive chemical and reduces the life of polymers and starches which are added to increase the viscosity of treating fluids of various types which are injected into the well. Finally, if hydrogen sulfide escapes into formations around the well, water contamination and the like can occur.
These and other problems are well known and a wide variety of scavenging agents have been used in an attempt to convert the hydrogen sulfide into non-toxic chemicals. For example, a recent article entitled "Chemical Scavengers for Sulfide in Water-Based Drilling Fluids" in the June, 1979 Journal of Petroleum Technology discusses the problem of hydrogen sulfide drilling and the reaction chemistry of commercial scavengers presently available for water-based drilling fluids. It states that there are three basic classes of scavengers in use at the present time, namely copper-based chemicals, zinc-based chemicals and iron-based chemicals. An example of a copper-based scavenger is copper carbonate. An example of an zinc-based scavenger is zinc carbonate. An example of an iron-based scavenger is iron oxide. Each of the scavengers has serious problems such that, as stated by the article, "we conclude that no commercial scavenger now is satisfactory for current needs."
While chlorine dioxide has been used in reducing hydrogen sulfide in well fluids, this use has largely been abandoned because of serious disadvantages and hazards in adding chlorine dioxide to the well fluids. This hazard stems primarily from the fact that chlorine dioxide is a toxic gas and is spontaneously explosive in concentrations above 10% in the atmosphere. Accordingly, it is extremely dangerous to produce, transport and use.
For example, U.S. Pat. No. 4,077,879 to Smeck, describes a process for purifying aqueous solutions containing hydrogen sulfide wherein chlorine dioxide is added to the solution in specific amounts and rates to avoid formation of colloidal sulfur. It states that one use for this process is in reducing hydrogen sulfide in well fluids. In the Smeck process, chlorine dioxide gas is added to the subject liquid to reduce the sulfide in the liquid. This is not satisfactory, however, since it requires generation systems for chlorine dioxide to be located at the well site. Alternatively, it requires the chlorine dioxide gas to be transported to the well in a gas container and then metered through a gas metering system into the well fluid at a specified rate. Here again, this method is so dangerous that it is operationally not feasible. Furthermore, this method does not add sufficient amounts of chlorine dioxide to the drilling fluid such that hydrogen sulfide gas can be prevented from reaching the surface. Since this is the most dangerous aspect of the presence of hydrogen sulfide gas, the Smeck patent fails in the most important regard. This failure is due to the inability to dissolve a large enough proportion of chlorine dioxide in the fluid. The present invention overcomes this problem.
Other oxides such as hydrogen peroxide have been used in reducing the hydrogen sulfide content in steam or drilling steam as shown in U.S. Pat. Nos. 4,163,044 and 4,151,260 to Woertz. However, hydrogen peroxide acts in a different manner than chlorine dioxide in oxidizing hydrogen sulfide in drilling mud.
Although stabilized chlorine dioxide solutions have been used for reducing bacteria in water and the like, they have not been used for reducing hydrogen sulfide content in well fluids. Examples of methods for making aqueous chlorine dioxide solutions are shown in U.S. Pat. Nos. 2,701,781, 3,271,242 and 3,278,447. For purposes of reference, the disclosure of these three patents is hereby incorporated in this disclosure. These three patents describe methods and compositions which allow chlorine dioxide solutions to contain chlorine dioxide in the range of 2% to 6% by weight of the solution. By "potential" chlorine dioxide it is meant that the solution can produce free chlorine dioxide when added to a solution wherein an oxidation-reduction reaction occurs.